The present invention relates generally to the field of semiconductor manufacturing and more particularly to a process of forming and using a lithographic mask.
In the field of semiconductor manufacturing, the ever smaller dimensions required to make state-of-the-art semiconductor devices are rapidly approaching the limits of optical lithographic techniques. Alternatives are being investigated to replace optical lithography once the resolution and depth of focus limits of optical lithography tools are reached. One lithographic technique that is being investigated is projection electron beam (E-beam) lithography. One implementation of electron beam lithography is referred to as scattering with angular limitation in projection electron beam lithography, also referred to as SCALPEL.
SCALPEL masks are basically comprised of a thin, low atomic number membrane material which is fairly transmissive to electrons. On top of these membranes, patterned high atomic number scattering material is formed. In order to fabricate the mask, portions of the silicon substrate are removed to form free-standing membranes. The membranes have width and length dimensions that are approximately 1.1 mmxc3x9712.1 mm. Using conventional wet etch processes to form the membranes on a 200 mm silicon substrate having a (100) crystal orientation, only approximately 528 membranes (an array of 8xc3x9766 membranes) can be formed. This presently corresponds approximately to a semiconductor die having dimensions of approximately 24xc3x9716.5 mm. As semiconductor devices become more complex, chip die dimensions of 25xc3x9725 mm will likely be required and an array of 10xc3x97100 (1000 membranes) will be needed.
Turning to the drawings, FIG. 1 is a cross sectional view of a mask 10 used for a SCALPEL process. The mask 10 includes a membrane layer 104 formed overlying a substrate 102. Typically, the membrane layer 104 is silicon nitride and the substrate 102 is monocrystalline silicon. On one surface of mask 10, scattering elements 108 are formed overlying membrane layer 104. On the opposite surface of mask 10, portions of substrate 102 are removed to form voids 110 and struts 103. The voids 110 define the regions 109 of membrane layer 104 over which scattering elements 108 may be formed. The scattering elements define the lithographic patterns used for forming the semiconductor devices.
One problem in manufacturing SCALPEL masks is that the formation of struts 103 is typically achieved using a wet etch process that results in a sloped strut sidewall indicated by reference numeral 111. The sloped sidewall 111 is characteristic of isotropic wet etch processes when etching semiconductor substrates with a (100) crystal orientation. Unfortunately, sloped sidewalls 111 limit the area of membrane layer 104 that can be used to form overlying functional scattering elements. This limitation translates into a corresponding limitation on the maximum die size that can be produced using mask 10.
To avoid the problems associated with sloped sidewall 111, one alternative in the formation of SCALPEL masks is to utilize a dry etch process to form the voids 110 in substrate 102. Unfortunately, however, dry etch processes typically require an etch stop layer formed between substrate 102 and membrane layer 104. The etch stop layer is typically required because of the lack of sufficient selectivity between substrate 102 and membrane layer 104 associated with dry etch processes. The introduction of additional processing required to fabricate the etch stop layer adds to the cost and complexity of the process required to form the mask. Further, the additional handling can result in an increased number of defects that can limit the yield of the mask formation process. In addition, if it is not subsequently removed, the presence of an etch stop layer during the fabrication of semiconductor devices can limit the throughput due to the loss of transmission of electrons through the additional layers on the mask. Furthermore, the deposition of the etch stop layer must be optimized to ensure that the stress associated with the etch stop layer matches the stress of the membrane layer thereby adding additional complexity to the process. Accordingly, it is highly desirable to implement a mask fabrication process that eliminates the sloped sidewall associated with conventional wet processing while minimizing the additional cost and complexity of the process.